This invention concerns a centrifugal pump for pumping liquids containing solid bodies, such as rags and other elongated objects.
A centrifugal pump, which sucks the liquid into its center and throws it outwards in the direction of the periphery, has no problems when pumping clean liquids. However, when the liquid contains solid bodies, the problems arise due to the fact that the bodies may get stuck between the pump impeller and the surrounding pump housing, causing a stoppage or at least a decreased pumping action.
One way to solve this problem is to design the pump in such a manner that the impeller rotates beside the flow itself, a so-called vortex type impeller. In this manner the clogging problems may be essentially removed, however, there are other disadvantages, such as a decreased pump efficiency.
Another method used to solve this problem, is to arrange some form of cutting means in front of the impeller which cuts rags and the like, so as to make it possible to pump these bodies. An example of such a solution is shown in the U.S. Pat. No. 4,108,386.
In the known construction the front end of the impeller is provided with two cutting means which cooperate with the inlet of the impeller for cutting the pollutions. However, the under pressure in the pump inlet enables an incoming rag to get sucked into the center of the impeller. Smaller rags may then rotate together with the impeller without making contact with the cutting means. Bigger rags may cause an increasing rotary resistance, sometimes to such a degree that the pump stops.
Another known construction is shown in the Swedish published patent application No. 444,969. Here a central body with cutting means is arranged, which body fills up the space in the inlet, thus diminishing the disk for one rags being sucked into the impeller center. This solution normally works very well, but under certain conditions a rag may stick to the center of the impeller and get stretched outwards to the periphery between the impeller and the housing. The friction which derives from that might lead to an overload of the motor and release of the motor protection device.
The two described impellers are similar in that they are both a so-called open type. This means that the impeller has only one cover disc on which the vanes are mounted and that the impeller is open towards the cutting means. The prevailing opinion has been that only open impellers are possible to use when pumping strongly polluted liquids. But, the closed impeller, which is provided with two cover discs and vanes therebetween, has an advantage in that its efficiency is far better than that of an open impeller.
According to the invention, use of a closed impeller when pumping strongly polluted liquids has been made possible. This possibility allows better efficiency and a longer bearing life. In addition it provides a better cutting result, thus diminishing the risks of clogging and damage.